I won't crossfire so that's out the way
500W (or greater) power supply with one 75W 6-pin PCI Express power connector recommended
Anyway PSU's are expensive and so I don't know what to get, I know I want modular, and would like to spend 80$ or less, but maximum 100$
What is it that you think modular will do for you ? Most of the better PSU's are what is called "hybrid modular" .... that is all absolutely necessary cables are hard wired to maximize efficiency and reduce the number of failure points. The rest or "optional" cables are modular.
Full modular provides absolutely no benefit whatsoever since you can not run a system w/o for example a 24 pin MoBo cable ..... and has a downside as the additional connector increases cost, decreases efficiency and introduces a failure point.
To find a decent, semi modular unit from a reputable brand at your budget doesn't leave a lot of choices ..... looking at today's newegg prices, I'd recommend the Antec HCG 620 is $50 after $20 MIRhttp://www.newegg.com/Product/Product.aspx?Item=N82E16817371059
The 900 watter was reviewed by jonnyguru here but id caution about reading too many parallels across the line.http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story5&reid=211